Detective Inspector Paul Hughes told reporters this morning the arrest was a "significant step forward" in a "highly complex and very sensitive investigation."

"We are doing everything we possibly can to try to establish in detail what has led to these baby deaths and collapses," he said.

Det Insp Hughes said there was "no set timescales for this coming to a conclusion but we remain committed to carrying out a thorough investigation as soon as possible".

There are very few details on the woman who has been arrested. Picture: Getty (Getty)

"Parents of all the babies are continuing to be kept fully updated and are being supported," he said.

"This is an extremely difficult time and it is important to remember that there are bereaved families seeking answers as to what happened to their children."

After the spike in deaths the hospital stopped delivering babies before 32 weeks of pregnancy, transferring the expectant mothers to other hospitals.

Before police were called in, a review by the Royal College of Pediatrics and Child Health found staffing gaps at the hospital for high-dependency and some intensive care patients but no definitive explanation for the baby deaths.

Hospital medical director Ian Harvey said "asking the police to look into this was not something we did lightly, but we need to do everything we can to understand what has happened here and get the answers we and the families so desperately want."